Go for the 90, you will love it

If you want top spend the money, an OS 91 Surpass (four stroke) turning a 15X6 APC prop is a perfect combo and with the stock tank flight times at half throttle are in the 20 minute range after the engine is broken in. I use mine with this combo and if I shoot touch and goes for a full tank, I can usually stay up for 30 minutes, no kidding, but that's on a well run engine.

I have a flying friend with the ST75 in his 60 and while it's nice, my Surpass out flies it easily. He has flown mine and I have flown his and the surpass is much nicer. If you want a foru stroke then Magnum makes a 91 that is really nice and fairly cheap and once broken in is a very reliable engine that will turn the same prop the Surpass does.

Here is a video of mine with the surpass being flown by a 15 yr old, as you can see it's definitely not lacking in power...

http://www.hiddenhangarrc.com/video/...xpert_test.wmv